Recently, an MEMS (Micro Electro Mechanical Systems) device using a silicon material for measuring a biochemical reaction has become a focus of attention.
For example disclosed has been a technique of providing plural through holes in a cell holding substrate, making a sample cell closely adhere to an opening of the through hole, and measuring a potential-dependent ion-channel activity of the sample cell with a measurement electrode arranged below the through hole.
Further disclosed has been a technique of forming a 2.5-μm through hole (hole) inside a cell holding substrate (membrane) made of silicon oxide, and making this through hole hold HEK293 cell as a kind of human cultured cell lines, to ensure high adhesiveness and measure an extracellular potential with high accuracy (e.g., refer to Non-Patent Document 1).
For such a structure used for such a cell holding substrate, a silicon material broadly in use in the field of the semiconductor technology is preferably used from the viewpoints of processability and productivity.
The surface of the silicon material used for a device made up of such a structure preferably has hydrophilicity or water retentivity, and in some cases, it is required to have both the hydrophilicity and the water retentivity. For the purpose of making the surface of the silicon material hydrophilic, a technique of forming a thin film of an inorganic oxide on the surface of the silicon material by sputtering has been disclosed (e.g., refer to Patent Document 1).
However, with the conventional configuration, imparting the hydrophilicity has been possible, but imparting the water retentivity has not been possible. Further, although it is required to form an area having the hydrophilicity or the water retentivity in a restricted specific area, it has been particularly difficult to form the area having the water retentivity in a restricted area.
Incidentally, living matters including humans organize a variety of cells to conduct activities. As a mechanism for transmitting stimulus information, received by a cell (e.g. visual cell) in some tissue from the outside, to another tissue cell (e.g. nerve), ion channels are present as a kind of functional proteins. These ion channels reside in cell membranes of every kind, and undertake an important role of allowing passage of ions (e.g. Na+, K+, Ca2+, etc.) between the inside and outside of the cell, to generate a current to be transmitted between cells or a potential difference.
In recent years, it has become possible, by finding out details of actions of these ion channels, to measure an effect of a new medicine at a cellular level or to measure the presence or absence of a side effect. While a variety of methods for measuring ion channels are present, a patch clamp technique has been the most used method since being capable of accurately measuring actions of ion channels in a single cell. Among the back clamp techniques, a planar patch technique capable of holding cells on a plane substrate has been a great focus of attention as being effective in increasing a throughput of measurement.
In this planer patch technique, a cellular electrophysiological sensor is used as a sensor portion for holding and electrically measuring a cell. As a sensor chip for this cellular electrophysiological sensor, the foregoing structure using the silicon material (hereinafter referred to as a “silicon structure”) can be employed.
An example of conventional cellular electrophysiological sensors is described in further details. FIG. 37 shows a sectional view of a conventional cellular electrophysiological sensor. As shown in FIG. 37, sensor chip 201 for the cellular electrophysiological sensor is provided with thin plate 203 having conduction hole 202, and frame body 204 arranged on this thin plate 203, and has cavity 205 inside frame body 204. Further, these thin plate 203 and frame body 204 have been processed using silicon material with high accuracy.
The cellular electrophysiological sensor using this sensor chip 201 is provided with: chip holding plate 206 with sensor chip 201 inserted therein; electrolytic baths 207, 208 arranged above and below sensor chip 201; and electrodes 209, 210 respectively arranged inside these electrolytic baths 207, 208.
In this cellular electrophysiological sensor, each of electrolytic baths 207, 208 is filled with an electrolyte, and cells 211 are then injected into upper electrolytic bath 207. Subsequently, by absorbing the electrolyte or the like downward from lower electrolytic bath 208 or performing some other operation, cell 211 can be captured at an opening of conduction hole 202. A potential difference between electrolytic baths 207, 208, a current, a resistance or the like can then be measured, so as to measure physicochemical changes during activities of cells 211, namely actions of ion-channels.
Here, it is required in the measurement that each of top and under surfaces of sensor chip 201 be filled with the electrolyte. However, since the surface is made of a hydrophobic silicon base, it is difficult to fill the inside of cavity 205 with the electrolyte. Therefore, as a method for rendering sensor chip 201 hydrophilic, there exists a method of thermally treating sensor chip 201 to form hydrophilic thermally oxidized film 212 on the surface of the silicon base.
It is to be noted that a similar example to above sensor chip 201 is disclosed in Patent Document 2 mentioned below.
However, there has been a problem with conventional sensor chip 201 in that measurement accuracy of the cellular electrophysiological sensor may decrease.
The reason for this is that bubble 213 may be generated inside cavity 205 of frame body 204 depending upon a difference in environment where measurement is performed.
Specifically, even when thermally oxidized film 212 is formed on an inner wall of frame body 204, an organic matter or the like adheres to the surface of the film with time, to lower the hydrophilicity. This makes bubble 213 apt to be generated inside cavity 205, and due to the presence of bubble 213, electrical conduction between the above and below conduction hole 202 is inhibited, or infiltration of a medicine is inhibited. As a consequence, there has been a problem in that the measurement accuracy of the cellular electrophysiological sensor decreases.
Moreover, another example of the conventional cellular electrophysiological sensors is described in further details. FIG. 38 shows a sectional view of a conventional cellular electrophysiological sensor. As shown in FIG. 38, sensor chip 301 for the conventional cellular electrophysiological sensor is provided with thin plate 303 having conduction hole 302, and frame body 304 arranged on this thin plate 303, and these thin plate 303 and frame body 304 have been processed using the silicon material with high accuracy.
Cellular electrophysiological sensor 305 using this sensor chip 301 is provided with: chip holding plate 306 with sensor chip 301 inserted therein; electrolytic baths 307a, 307b arranged above and below sensor chip 301; and electrodes 308a, 308b respectively arranged inside these electrolytic baths 307a, 307b. 
In this cellular electrophysiological sensor 305, each of electrolytic baths 307a, 307b is filled with an electrolyte, and cells 309 are then injected into upper electrolytic bath 307a. Subsequently, by absorbing the electrolyte or the like downward from lower electrolytic bath 307b or performing some other operation, cell 309 can be captured at an opening of conduction hole 302. A potential difference between electrolytic baths 307a, 307b, a current, a resistance or the like can then be measured, so as to measure physicochemical changes of cells 309 during activities of cells 309, namely actions of ion-channels.
Here, it is required that each of top and under surfaces of sensor chip 301 be filled with the electrolyte. However, since the surface is made of a silicon base that is apt to be hydrophobic, a bubble may be generated on the under surface of sensor chip 301. As a method for rendering the surface of sensor chip 301 hydrophilic to prevent generation of this bubble, there exists a method of thermally treating sensor chip 301 to form hydrophilic thermally oxidized film 310 on its surface.
It is to be noted that a similar example to above sensor chip 301 is disclosed in Patent Document 1 mentioned below.
However, there has been a problem with conventional sensor chip 301 in that measurement accuracy of cellular electrophysiological sensor 305 may decrease.
The reason for this is that a bubble may be generated on under surface 303a of thin plate 303 depending upon a difference in environment where measurement is performed.
Specifically, even when thermally oxidized film 310 is formed on under surface 303a of thin plate 303, an organic matter or the like adheres to the surface of the film with time, whereby the hydrophilicity decreases and bubble 312 is then generated. When, as a consequence, this bubble 312 adheres to the vicinity of lead-out port 311 of conduction hole 302, electrical conduction between above and below conduction hole 302 is inhibited.
Consequently, there has been a problem in that the measurement accuracy of cellular electrophysiological sensor 305 decreases.    [Non-Patent Document 1] “Micro Total Analysis Systems 2004”, T. Sordel et al., pp-521-522 (2004)    [Patent Document 1] Unexamined Japanese Patent Publication No. 2000-243700    [Patent Document 2] Unexamined Japanese Patent Publication No. 2004-69309